Cannot get multiple VLANs to work on one port for one PC on an Cisco SG300 switch.

I have a guest network that was setup kinda odd (not by me).
The management part of this network is on VLAN99 and runs in the 192.168.1.x address space.
This address space includes all the switches, access points, access point controllers and one management PC.

The guest part of this network is on VLAN100 and runs in the 192.168.111.x address space.
The Cisco WLC controller gives out DHCP addresses in the 192.168.111.x space and uses the gateway address of 192.168.111.1, which is a Sonicwall firewall (which is managed by Spectrum business solutions... don't ask).

If you login to the management PC you can manage the WLC and all the switches just fine.
However, because you are on VLAN99 and in the 192.168.1.x address space you cannot get to the internet.

If you login tot he WIFI you cannot manage any hardware but you can get to the internet.

This IS how it is supposed to work.

However, I would like to monitor the firewall, via SNMP, from my management PC on the 192.168.1.x network.

I have assigned a 192.168.111.x address to the PC as it's main address.
I have assigned VLAN100 as an additional VLAN on the port that it's plugged into.
But, it does not work.

The switch that I'm plugged into is a Cisco SG300-10SPF

The port I'm using is configured as:

interface gigabitethernet8
 description "Management PC Link"
 switchport trunk allowed vlan add 100

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On the 3650 line of switches that would allow VLAN99 (the default VLAN on the switch) and VLAN100 to use port 8.
On this switch... it just doesn't work.

I'm open to ideas.